When LNG is burned, the LNG produces a small amount of emission of nitrogen oxide and sulfurous acid gas and hence has been increasingly demanded as clean energy year by year. The LNG is produced by cooling natural gas to approximately −162° C. to thereby liquefy the natural gas and is carried by the sea to a consumption area by an LNG carrier.
Under circumstances where the price of energy is rising worldwide, development projects of a large-scale gas field of seabed far from land are currently beginning in earnest.
In the light of this trend, the following production and transportation system has been focused: that is, a marine floating system having a liquefied natural gas plant and an LNG storage tank arranged thereon is floated on the sea; in the marine floating system, the impurities of the natural gas are removed and the natural gas is liquefied to produce LNG and the LNG is stored in the tank; and when an LNG carrier arrives at the marine floating system, the LNG is offloaded (shipped off) from the LNG storage tank.
As compared with a case where a liquefied natural gas plant is constructed on the land, the transportation system has the following advantages: pipeline facilities from a gas field of seabed and in the sea to the land can be reduced; an environmental load can be reduced because development on the coast is not required; and workers can be comparatively easily secured because an LNG-FPSO is constructed in a country or a region different from those in which a gas field is developed and is towed to the site.
The LNG-FPSO (Floating LNG Production, Storage and Off-Loading system) used for this has the functions of liquefying gas, which is produced from a gas field of seabed, on the ocean to produce LNG, storing the LNG in a tank, and loading it onto an LNG carrier.
An LNG-FSRU (Floating LNG Storage and Re-gasification Unit) has a function of gasifying LNG received from an LNG carrier delivering the LNG gas to the land.
The LNG-FPSO include a tank or tanks for storing a large amount of produced LNG, and as its tank structure, LNG tank technology, which has been fostered in the construction of a conventional LNG carrier, is expected to be adopted. However, since how the LNG storage tank is used is different between the LNG-FPSO and the LNG carrier, care is needed. In the case of the LNG carrier, a phenomenon that a liquid cargo in the tank violently sloshes (sloshing phenomenon) is unlikely to occur even during heavy weather because the LNG storage tank is used either in full load condition or in unload condition and is never in half load condition. Only during cargo handling work, a liquid level in the tank greatly changes, but since the cargo handling work has been usually performed in a port where waves and winds are quiet, it has been possible to almost disregard the sloshing.
On the other hand, in the LNG-FPSO, the sloshing phenomenon is thought to be likely to occur because it is constantly moored on the ocean where a weather condition is severe and a liquid level in its LNG storage tank changes from time to time according to a production amount of LNG and a loading amount to the LNG carrier, and half load condition daily occurs.
Another important thing about the LNG-FPSO is that loading the liquid cargo to the LNG carrier with the use of a loading arm, a special hose with a coupling system or the like, by STS (ship to ship) operation, in particular, while the LNG carrier is alongside the LNG-FPSO (side by side), is now under consideration. Considering that the cargo handling for a conventional LNG carrier has been performed while the LNG carrier is moored at a berth provided in a safe port, it is thought that the aforesaid STS cargo handling on the ocean has a high risk caused by a collision accident occurs between the LNG-FPSO and the LNG carrier trying to approach it to damage the hull, or an accident such as the damage of the hull by leakage of the liquid cargo from the loading arm. Therefore, in designing the tank and ship of the LNG-FPSO, it is necessary to take such risks into full consideration.
Following three kinds of LNG storage tanks conventionally used in LNG carriers have been adopting, a self-supporting spherical tank (MOSS type tank), a self-supporting prismatic tank (SPB type), and a membrane tank and it is expected that one of these three tank types will be adopted also in the LNG-FPSO.
Regarding the self-supporting spherical tank, it is a self-supporting tank made of an aluminum alloy and is supported in a hold of a double hull construction, via a skirt extending from its equatorial portion. A thermal insulation layer is applied on an outer surface of the tank (external thermal insulation). Due to its spherical shape, the self-supporting spherical tank has a disadvantage of low volumetric efficiency because it is not well fitted in the hold. However, in the tank of this type, owing to its external thermal insulation, the thermal insulation layer does not suffer damage even by sloshing during heavy weather.
Regarding the self-supporting prismatic tank, a main body is a prismatic tank made of an aluminum alloy and longitudinal strength members reinforcing of the tank are provided on an inner side of the tank, and a thermal insulation layer is provided on an outer surface of the tank. This type requires void space between the prismatic tank and an inner hull of the ship, which accordingly reduces volumetric efficiency of the tank. On the other hand, since the longitudinal strength members can be provided inside the tank, sloshing of a liquid cargo does not easily occur during heavy weather, and even if the sloshing occurs, the thermal insulation layer provided on the outer surface of the tank is not damaged.
In the membrane tank, on an inner surface of a hold fabricated with a double hull structure, thin sheets (membranes) made of nickel steel or stainless steel are affixed, with a thermal insulation layer there between, to form a LNG tank. This type is excellent in volumetric efficiency because almost all the volume of the hold can be used as a tank volume. On the other hand, it has a disadvantage that the membranes and the thermal insulation layer are likely to be damaged by the sloshing of a liquid cargo. It also has a problem that a thermal insulation work, in particular, the welding of the membranes is complicated and it requires a long period for the construction.